One River North: Where City and Nature Take Form

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Denver, Colorado, is known as a city where residents enjoy a connection to the outdoor lifestyle due to the city’s proximity to the Rocky Mountains. One River North, one of Denver’s newest buildings, was designed to reflect that connection and give residents a place to call home with the convenience of the city and feel of the mountains. World-renowned architecture firm MAD Architects based in Beijing, China, dreamed up a building design that would provide a mountain-like feel in luxury urban housing.

The building stands out as unique due to the prominent canyon-like features that meander across the building face and are meant to resemble a structure that has been overtaken by nature. The canyon features make the design unlike any other, which forced the construction and design teams to come up with collaborative and creative solutions for the unique geometry. The design team consisted of MAD Architects, who provided the overall architectural design, Davis Partnership Architects who served as local Architect of Record and who also provided the interior and landscape design, Jirsa Hedrick who performed the structural design, KHSS who provided the design of the canyon plaster system, and many others. Saunders Construction Inc. served as the project general contractor and provided pre-construction design assist services.

One River North is 16 stories above grade with three stories of below-grade parking beneath. The ground level consists of retail space, the main resident entry lobby, resident amenities such as bike storage and pet wash, some mechanical rooms, as well as the porte cochere and access to the parking garage below. The three below-ground parking levels provide approximately 175 parking spaces for building residents. Levels two through sixteen are residential units, totaling over 180 units. The building boasts thousands of square feet of outdoor amenity spaces, most of which are found on levels six through eight where residents enjoy the sprawling canyon landscape with seating areas, fire pits, a walking path and waterfall. An additional outdoor amenity space on the roof provides residents with a rooftop pool, hot tub, seating areas and views of the mountains to the west.

The building foundations, structural walls, columns, and slabs are cast-in-place concrete. The building foundation system is made up of 76 drilled piers that extend from the basement level into the bedrock below with a typical total length of 35-45 feet. The piers range in diameter from 24 inches to 60 inches at the heaviest loaded columns. The pier cap below the building core walls has nine piers that provide the lateral resistance at the foundation, these piers are 48 inches in diameter with almost 2% vertical reinforcing.

Due to the unique geometry, the building has irregular column spacing, but an approximate 28 feet by 28 feet column grid is used where possible. The columns come in 19 different shapes, and some of the largest sections used include 36-inch diameter round columns and 26-inch by 48- inch rectangular columns. Round columns were preferred for aesthetics at outdoor spaces, but square and rectangular columns were preferred at interior applications. The columns that pass vertically through the canyon space had to be coordinated in many locations to account for the change in shape from rectangular to round and back. Columns were constructed with high strength 8000 psi concrete. The floors of the building are typically two-way, post tensioned slabs that are 8 inches or 9 inches thick in residential areas. The slab thickness is increased in areas with large column spacing or heavy landscape loads, with corresponding increased reinforcing. Slabs were placed with 6000-psi concrete. The building lateral system is provided by the single, box-shaped elevator core at the center of the building. The elevator core walls are 18 inches thick and utilize the 8000-psi concrete mix. The canyons that make the building look so unique are a special panelized plaster and mesh system, referred to as the CHIPS system, that mounts to steel tabs fastened to the structural slabs. The building cladding is a combination of stucco panels in the back and curtainwall system on the front facade.

The construction of the project began in 2021 with excavation of the existing site down three levels to the basement elevation and erection of the tower crane. Installation of the drilled piers and basement structure took approximately six months, with construction of the ground level slab in spring of 2022. The building was topped out just nine months later in December 2022. The building elevation was visibly different from a typical project during construction, but it wasn’t until the installation of the CHIPS system that the general public realized how unique the building was going to be. Installation of the CHIPS system began in fall of 2022 and wasn’t fully completed until late 2023 at which point the project began attracting increased local media attention. After just three years of fast-paced construction, the building was completed and opened to the public in April of 2024.

The CHIPS system that adorns the building uses the same technology used to build man-made mountains at theme parks and is designed and fabricated by a third-party specialty company. The system gets its name from its resemblance to a potato chip. Each of the CHIPS segments came out in hundreds of prefabricated mesh panels, typically 48 inches by 48 inches in size. The CHIPS panels were mounted to steel columns and braces that were fixed to embed plates cast into the structural slabs and columns. Once all panels are installed and tied together, they are coated with several applications of plaster and paint. The support system required the placement of thousands of embed plates prior to pouring the concrete slabs and columns, and there were zero missed embed plates through the entire course of construction. Ensuring all embed plates were placed and located correctly was achieved by TEKLA laser scanning technology and integration into the project Navisworks model. Missing or misplaced embed plates were flagged and corrected prior to placement of the concrete. The CHIPS unique geometry was modeled by the MAD Architects team in Rhino 3D software, and the Rhino BIM model was a contract document used by the CHIPS fabricator to produce the desired shapes and curves.

Simply attaching the CHIPS system to a flat-faced structure would not achieve the full desired slot canyon effect. Therefore, in many locations the curves of the canyon CHIPS geometry match up with the structure below to make a gash in the face of the building and create a penetrating reveal into the structure behind the cladding. Many of the final curves were modeled using Rhino splines—curves without definable geometry in 2D documentation. The edges of the structural slabs are defined in the structural drawings for typical locations but the structural team deferred the curved areas to the architecture team to convey due to the complexity. The architectural team plotted the curves on an X-Y grid system to document the unique geometry. The slab edges were then surveyed extensively to ensure the curved formwork aligned with the BIM models before placing the concrete. The general contractor utilized TEKLA’s Augmented Reality goggles which allowed the user to see an overlay of the BIM model while on site and verify if the curved formwork aligned with the design. Many of the curved areas were heavily reinforced to support the landscape loads, and thus a substantial amount of reinforcing would had to be field-trimmed to fit into the various corners and curves.

Another challenge in construction was the coordination of the concrete floor shoring and reshoring operations. In general, the structural team required a minimum of two floors of reshores in addition to one floor of formwork, requiring three floors total of support for each slab pour. The reveals in various parts of the canyon created conditions at some levels where there may not be a slab below for three levels and required the shoring in these areas be supported by slabs five or more levels below. The shoring coordination added another hurdle for the construction team to coordinate activities in the areas that would be blocked by shoring far longer than typical. In an effort to keep glass installation on schedule the structural team provided a loading allowance for limited construction activities in reshore zones.

A crack running through the structure is usually the last thing a structural team wants to hear about; in this case, the crack in the face of the structure was the defining element and couldn’t be glossed over. The building’s unique gash feature is accomplished by carefully curving the slab edges to mold to the shape of the canyon. The levels with canyon features, levels six through the roof, all have unique geometry and require unique reinforcing. The side of the building facing the alley—referred to as the backbar—was the same for levels three through 17. The building floors were constructed in two separate concrete pours, typically spaced a week apart allowing for a completed level every two weeks. Due to the complexity of the reinforcing and demanding overall project schedule the structural team was on site to review the reinforcing for almost every concrete pour which included weekend, sunrise, and sunset site visits throughout the course of construction.

Floor slabs were designed using RAM ConcePT post-tensioned slab analysis software. The floor geometry was exported from Revit for each level to properly analyze the slab edges and landscaping load geometry. In most instances the landscape loads were located near the edges of the floor slabs in cantilever zones so additional post tensioned reinforcing had to be placed to reduce slab edge deflection and provide the required capacity. While typical interior slab areas may only have 150 psi of precompression, the heavily-loaded landscape areas often have over 400 psi of precompression. The cantilevered western slab edge was also cambered as much as one inch in some locations to account for anticipated deflections from the cladding, landscape, and CHIPS weight (approximately 15-psf). Locations with extensive heavy landscaping, such as the level 6 waterfall feature, are thickened to 12 inches with additional heavy mild reinforcing.

One of the design challenges for the post-tensioned slab system included a three discontinuous columns that supported several levels above level 1 but did not continue to the basement below and had to be supported by a thickened transfer slab at level 1. The columns stop at level 1 and are supported by the 40-inch-thick PT slab below, with over 100 PT tendons in each direction and reinforcing mats and ties at 9-inches on center. The 24 inch by 36 inch discontinuous column has a design load of over 2000 kips that is transferred via the post tensioned slab to the columns over 10 feet away below. The construction of the transfer slab was also challenging because it required the rebar installers to crawl fully inside the rebar cage and lay horizontally to place the cross ties. Despite the large balanced load, enough mild reinforcing was provided to resist initial stresses and avoid having to use staged stressing.

One River North is located directly next to a large trainyard and several large parking lots which all contribute to a C exposure classification. The building is classified as Seismic Design Category B, so the wind loading controlled the lateral design over seismic effects. The building was studied with a wind tunnel analysis by local wind tunnel expert Cermak Peterka Peterson (CPP Wind) who provided the design team with MWFRS and components and cladding design loads. The building lateral analysis was performed using RAM Structural System with a lateral model calibrated to correlate with the wind tunnel criteria. The design team considered building drift, backstay effects, and system ultimate capacity with multiple RAM models that were used to envelope the various possible conditions.

The building lateral system is provided by the concrete elevator core walls which make up a 30 feet by 30 feet square at the center of the building. The core walls are 18-inches thick and have varying levels of reinforcing. A corridor passes through the elevator lobby and openings for the corridor are required on each side of the core. The link beams above the corridor openings at most levels are typically lightly loaded and easily designed with reinforced concrete sections. However, the lower levels with larger openings have shear stresses exceeding what is allowed by code with normally reinforced sections and required the design team to resort to an embedded W24x131 wide flange beam across the corridor openings at levels three through five. Embedding a steel beam with welded reinforcement inside the link beam formwork required extensive coordination with the construction team to ensure the beam could fit in the wall reinforcing mats. The first beams were placed at level three with minimal issues and the beams at levels four and five went in without any further coordination required.

At one time, One River North seemed like another project that would be doomed by the Covid 19 economic chaos and a prohibitive price tag. The design team was excited to take part in the project, but post-Covid economic uncertainty and fluctuating costs of construction meant a higher risk of project delay or budget constraint. Now the building is complete, and it is an undeniable example of Denver’s special architecture. One River North stands as a source of pride and accomplishment for all who worked on the project. For the structural team at Jirsa Hedrick, the project represented one of the most complicated ever undertaken as this project required a high level of collaboration, coordination, and creativity. By relying on expertise in BIM modeling and structural analysis software, the design team was able to navigate the difficult design and construction journey and successfully produce a design that blends nature with downtown Denver. ■

About the Author

Austin Reese, PE, is a project manager for Jirsa Hedrick. He has 10 years of experience in structural design and analysis. He has completed work on multiple projects including senior living, multi-family residential, commercial, and public structures.